In semiconductor device production steps, an exposure tool for transferring a fine circuit pattern onto a wafer to produce an integrated circuit has hitherto been widely used. In recent years, with the trend toward higher degrees of integration and function advancement in semiconductor integrated circuits, refinement of the semiconductor integrated circuits proceeds. In order to precisely form a circuit pattern image on a wafer surface, a glass substrate for a mask blank used as a photo mask for an exposure tool is required to have a high degree of flatness and smoothness.
Further, under such technical circumstances, a lithographic technique using EUV (extreme ultraviolet) light (extreme ultraviolet light) as a next-generation exposure light has attracted attention since it is considered to be applicable over a plurality of generations, which are the 45-nm and succeeding generations. The EUV light means a light having a wavelength in the soft X-ray region or vacuum ultraviolet region, specifically a light having a wavelength of about 0.2 to 100 nm. At present, it has been investigated to use a light having a wavelength of 13.5 nm as a lithographic light. The exposure principle of this EUV lithography (hereinafter abbreviated as “EUVL”) is equal to that of conventional lithography in that a mask pattern is transferred by using an optical projection system. However, a refractive optical system cannot be used because there is no material which transmits light in the EUV light energy region, which results in use of a reflective optical system (see patent document 1).
The photo mask used in EUVL is basically constituted of (1) a substrate, (2) a reflective multilayer film formed on the substrate, and (3) an absorber layer formed on the reflective multilayer film. As the substrate, a material having such a low coefficient of thermal expansion that causes no strain even under irradiation with the EUV light is required, and a glass or crystallized glass having a low coefficient of thermal expansion has been investigated. The substrate is produced by highly accurately polishing and washing a raw material such as the glass or crystallized glass.
In general, methods of polishing a substrate for a magnetic recording medium, a substrate for a semiconductor device, or the like to have a highly smooth surface are known. For example, patent document 2 discloses a method of performing polishing, in regard to finish polishing of a memory hard disk or polishing of a substrate for a semiconductor device, by using a polishing liquid composition comprising water, an abrasive material and an acid compound, having an acidic pH and having an abrasive material concentration lower than 10% by weight, as a polishing method for decreasing surface roughness of a material to be polished after polishing and reducing minute protrusions (convex defects). As the above-mentioned abrasive materials, aluminum oxide, silica, cerium oxide, zirconium oxide and the like are disclosed. As acids for making the pH acidic, nitric acid, sulfuric acid, hydrochloric acid, organic acids and the like are disclosed.
Further, patent document 3 discloses a method of polishing a glass substrate mainly comprising SiO2 by using a polishing slurry comprising colloidal silica having an average primary particle diameter of 50 nm or less and water and adjusted to have a pH ranging from 0.5 to 4, to thereby polish the substrate so as to have a surface roughness Rms of 0.15 nm or less. Furthermore, it is disclosed that the number of concave defects having a width of 60 nm or more can be decreased to 3 or less in the area of 142 nm×142 nm by this polishing method, and that 80 g/cm2 is used as the polishing load (Examples).
Patent Document 1: JP-T-2003-505891
Patent Document 2: JP-A-2003-211351
Patent Document 3: JP-A-2006-35413